Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photodetector being integrated within a pickup. The photodetector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store up to 50 GB on a dual layer disc. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. The re-writable BD-RE disc is based on a phase change technology comprising a phase change layer, which uses for example a compound of AgInSbTe or GeSbTe.
New optical storage media with a super-resolution structure offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-Ray disc. This is possible by a so-called super-resolution layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super-resolution layer is a nonlinear layer, which is also called mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, semiconductor materials can be used as a nonlinear layer, e.g. InSb, which show a higher reflectivity in the center part of the focused laser beam, the reflectivity being dependent on the pit structure of a corresponding data layer. Therefore, the super-resolution effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of a corresponding optical pickup. The super-resolution structure is often also called a super-resolution near-field structure (Super-RENS) because it is assumed that the super-resolution effect is based on a super-resolution near-field effect.
In the articles Hyot et al, “Phase change materials and Super-RENS”, E*PCOS05, Technical Digest, Cambridge, 2005, and Pichon et al, “Multiphysics Simulation of Super-Resolution BD ROM Optical Disk Readout” 2006 IEEE, 0-7803-9494-1/06, PP 206-208, a semi-conducting mask layer is proposed in which a local change of the refractive index can be obtained through photo generation of free carriers. A thermal description is given to provide information on temperature distribution during readout of a data layer.
In US20050153108 an optical storage medium is disclosed comprising a substrate layer, a data layer, a first and a second super-resolution layer used as mask layers and an insertion layer disposed between both mask layers. For the mask layers, oxides from the noble metals Pt, Au, Pd, or Ag are used. Optical storage media comprising at least two super resolution layers are also known from US2005259563, EP1492101 and U.S. Pat. No. 7,232,598. For nearly all of the metal oxide based optical storage media described in these references, the metal oxide is in particular used as a recording layer. The super-resolution effect is then partly based on a metal nano-particle effect and partly based on a phase-change effect.